"I don't think I feel very good, it's not about numbers," Monfils said. "I think I'm not in my best shape yet, I'm fighting for every point. Tomorrow [Friday] will be a great match, Schwartzman is a very fast player. I will have to play more aggressive."

Defending champion and second seed Thiem eased through to the quarters following the retirement of opponent Pablo Andujar, who was trailing 4-2 in the first set.

Next up for Thiem is eighth seed Fernando Verdasco after the Spaniard rallied past Nicolas Kicker 6-7 (3-7) 6-2 6-0.